* Add the 'ifconsole' option. This allows you to construct a line as shown
below to enable a getty (e.g. on a serial port) only if the tty is the
system console.

ttyd0 "/usr/libexec/getty std.9600" dialup on secure ifconsole

The ifconsole option also silently disables any devices which do not
exist, removing unnecessary console spam.

* Modify libc and init to handle the new flag.

* Modify the nrelease build and ttys defaults to use this option for
ttyd0, ttyd1, ttyd2, and ttyd3.

* Avoids previously reported issues with system crashes on certain machines
whos serial ports are not implemented properly without having to fully
disable the default ttyd* getty feature. If the serial port isn't the
console, it won't be enabled.

* Add a sysctl to the kernel which init needs to test whether a tty is the
console or not.